The signal bandwidth and channel spacing used for satellite communication is generally different from the signal bandwidth used in cellular systems. One reason for this difference is that satellite communication is limited by thermal noise. As a result, satellite communications uses lower bandwidth and coding rates. On the other hand, cellular communications is interference limited, favoring higher bandwidth and coding rates.
For example, the GSM cellular system's channel spacing is 200 KHz, while the INMARSAT-M satellite system uses 5 KHz or 10 KHz channel spacing. In the latter narrowband mode, frequency and phase noise is considerably more troublesome for moving terminals or mobile stations than in the former, wideband mode. There can thus arise difficulties in attempting for economy in the terminal to re-use circuitry for both modes.
U.S. patent application Ser. No. 08/305,780, which is incorporated herein by reference, describes a dual-mode terminal equipped with a novel frequency synthesizer circuit that economically provides both narrowband satellite channel spacings as well a GSM spacings while meeting the stringent noise requirements in a narrowband satellite mode and the fast switching requirements for the GSM frequency-hopping mode.
Other prior descriptions of dual mode terminals that economically re-use the same components between two modes may be found, for example, U.S. patent application Ser. No. 07/585,910 entitled "Multi-mode signal processing", which is incorporated herein by reference, describes re-using the same components to process alternatively an analog FM signal according to the AMPS cellular standard or a digital cellular signal according to TIA standard IS54.
The GSM standard discloses the possibility to transmit lower bitrates by transmitting TDMA bursts at the same bitrate only less often. The GSM standard describes a so-called "half-rate" mode in which a burst is transmitted only every 16 timeslots instead of every eight. However, the same format is used in the uplink direction (mobile to base) as in the downlink direction (base to mobile), which leads to problems of high peak power requirements from the mobile phone in a satellite system.
U.S. patent application Ser. No. 08/179,954, which is incorporated herein by reference, discloses asymmetrical TDMA formats in which uplink TDMA formats can have a smaller number of timeslots combined with a greater availability of narrower bandwidth frequency channels than the corresponding downlink TDMA formats, thus reducing the peak-to-mean power ratio needed in the mobile terminal. When practicing the invention disclosed in the above incorporated application however, a terminal is not capable of being compatible with the GSM cellular standard's uplink waveform.